The present invention relates to a centrifugal fan that collects airflow taken in from an air inlet formed at the center of one flat base wall of a scroll casing and discharges the airflow from an exhaust port formed on a cylindrical side wall in a centrifugal direction. More particularly, the present invention relates to a mechanism to prevent backflow during fan halting.
Centrifugal fans, which use DC brushless motors especially, are widely used to cool electronic components of OA equipment such as a personal computer, a copying machine, a liquid crystal projector and a disk array because they can not only make the motors compact and light in weight but also control air quantity easily due to easy control of the motor. A plurality of centrifugal fans are used when a target device has a high heating value.
Some centrifugal fan shave backflow prevention mechanisms to prevent backflow from the exhaust port to the air inlet during fan halting due to faults of motors or driving circuits. For example, Japanese unexamined patent publication No. 2003-278697 (JP2003-278697A) discloses a centrifugal fan that has the backflow prevention mechanism.
The backflow prevention mechanism of the centrifugal fan disclosed in the publication is provided with a flat-shaped flap located in the airflow path to the exhaust port of the scroll casing to shut an airflow path during fan halting. The flap is rotatably supported at a base wall of the casing that becomes upper side in a vertical-use position where a rotating shaft of a fan motor is vertical. In the vertical-use position, the airflow pushes the flap to turn open and is discharged from the exhaust port during fan operating. When the fan stops, the flap turns to shut the airflow path under its own weight and prevents backflow.
However, since the centrifugal fan disclosed in the above-described publication is designed on the assumption that it would operate in the vertical-use position, the backflow prevention mechanism may not function when the fan is arranged so that the rotating shaft of the fan motor is horizontal due to limitations of space for example. That is, the rotation axis of the flap is almost perpendicular to the airflow discharging direction. This arrangement has an advantage in minimizing the flap area. However, when the centrifugal fan is arranged so that the rotation shaft of the fan motor is horizontal (a horizontal-use position) and the airflow discharging direction is also horizontal, the rotation axis of the flap becomes almost vertical, which disables the flap from closing by its own weight.